Conventionally, a variable geometry turbocharger in which a movable nozzle vane is provided to a nozzle of an exhaust turbine and the nozzle vane is rotated to adjust an opening degree of the nozzle (i.e., opening area of the nozzle) is known. With the variable geometry turbocharger, at a low speed revolution zone of an engine having a small displacement, the opening degree of the nozzle is reduced by rotating the nozzle vane to increase a flow speed of exhaust gas flowing into the exhaust turbine, thereby increasing the rotary energy of an exhaust turbine wheel to enhance supercharging performance of a charging compressor.
Known specific structures for rotating the nozzle vane include a structure in which one of a plurality of nozzle vanes is connected to a driveshaft, rotation of the driveshaft being allowed to be actuated from an outside, and a drive lever is attached to the driveshaft. The drive lever rotates a subordinate lever provided to another of the plurality of nozzle vanes via a connector ring. With this arrangement, all of the nozzle vanes can be rotated by rotating one nozzle vane by the driveshaft. (e.g., Patent Document 1)
Also, according to Patent Document 1, the driveshaft connected to the nozzle vane is actuated by a pneumatic actuator that uses negative pressure of intake passage. Here, the pneumatic actuator includes a housing having a negative pressure chamber to which the negative pressure is introduced from the intake passage and an atmospheric pressure chamber opened to the atmosphere. The chambers of the housing are partitioned by an operational plate (diaphragm) that operates in correspondence with a value of the negative pressure. The operational plate is provided with a rod, which advances and retreats in correspondence with movement of the operational plate. The advancing and retreating movement is converted to rotary movement of the driveshaft to adjust the opening degree of the nozzle.
On the other hand, employment of a hydraulic servo actuator of the four port type instead of the pneumatic actuator has also been proposed (e.g., Patent Document 2). According to Patent Document 2, a mechanism for a variable opening degree of the nozzle is actuated by a hydraulic servo actuator, thus achieving a more precise control of the opening degree. The hydraulic servo actuator switches the supply of the pressure oil to the hydraulic chambers on both sides of the servo piston by a proportional solenoid valve. In other words, a position of a spool forming the solenoid valve is switched to switch the supply of hydraulic pressure to the hydraulic chambers.    Patent Document 1: JP-A-11-343857    Patent Document 2: JP-T-2003-527522